Vascularization of many solid tumors occurs in vivo as a consequence of the vectorial growth of host capillary endothelial cells along a concentration gradient of a tumor-produced, diffusible angiogenesis factor (AF). Our studies of this process employ an angiogenesis assay in rabbit cornea and two in vitro assays that examine effects of tumor-derived agents on the growth and migration properties of cultured endothelial cells (EC) from fetal bovine aorta. Chromatographic and spectroscopic methods have been employed in identifying angiogenic substances present in ethanol extracts from lyophilized cell homogenates of the Walker 256 rat tumor. These substances, adenosine, inosine, and hypoxanthine, are markers for hypoxic or ischemic tissues and may help to explain the general observation that tissue oxygen deprivation precedes neovascularization. In vitro studies reveal these substances to be chemotactic but not mitogenic for vascular EC. The presence of EC mitogenic material in the tumor preparations indicates that other low molecular weight potential AFs need to be characterized. Partially purified mitogenic material has been examined by 1H-\and 13C-nuclear magnetic resonance without producing a definitive structure. A potential antiangiogenic agent, protamine, inhibits EC growth when stimulated by a polypeptide EC growth factor derived from bovine retina. This growth inhibition is blocked by including heparin, pentosan polysulfate, or dextran sulfate in the test medium. Cell growth initiated by the low molecular weight tumor-derived AF is unaltered by addition of protamine to the test medium suggesting that angiogenesis is regulated by a variety of specific tissue and cellular factors. Future studies will seek: (1)\to complete studies on the structure determination of the low molecular weight EC mitogen and AF from the Walker rat tumor; (2)\to conduct a comparative survey of the angiogenic activity and AF molecules present in tumors that in situ differ in degree of vascularization; (3)\to examine the chemoattractant, motogenic, and angiogenic properties of analogs and derivatives of identifiable tumor AF, including the adenine nucleotide catabolites that act as EC chemoattractants and the mitogenic factors for vascular EC; and (4)\to examine the effects of various basic and acidic biopolymers on vessel growth in in vivo assay systems. We hope to provide improved means for controlling tumor growth and spread and for detecting solid tumors. (J)